Process for forming crosslinked oriented, microporous polyolefin film



3,376,238 PRGCESS FOR FORMING CROSSLINKED DRIENTED, MICROPOROUS POLYOLE-FIN Fi'LM Razmic S. Gregorian, 8460 Piney Branch Court, Silver Spring,Md. 20901, and Charles C. Kirk, 2808 Johns Hopkins Road, Laurel, Md.20810 No Drawing. Continuation-impart of application Ser. No. 337,186,Jan. 13, 1964-. This application May 19, 1966, der. No. 551,248

4 Claims. (Cl. 260-25) ABSTRACT OF THE DISCLOSURE A process for formingcrosslinked biaxially oriented, microporous, polyolefin-containing filmcomprising admixing said polyolefin with a finely divided pore formingsolid, heating the mixture above the melting point of the polyolefin toshape same in the form of film, crosslinking the shaped mixture byeither adding an organic compound capable of generating free-radicals oremploying ionizing irradiation and thereafter extracting the poreforming solid at a temperature below the degradation temperature of thecrosslinked polyolefin. The polyolefin film can, if desired, bebiaxially oriented either after the crosslinking step or after theextraction of the pore forming solid.

This application is a continuation-in-part of our prior copendingapplication Ser. No. 337,186 filed I an. 13, 1964 now abandoned.

This invention relates to a novel and useful composition of matter, aprocess utilizing the composition and the product resulting from theprocess. More particularly, it is directed to filled polyolefincontaining compositions, e.g. polyethylene, polypropylene and copolymersof ethylene and vinyl monomers, a process employing said compositionsand the microporous product resulting therefrom.

It is known in the art to render thermoplastic polymers, microporous byincluding therein a pore-forming solid which is subsequently removed.The pore-forming solid is extracted at low temperatures which do notcause physical or chemical changes, e.g. swelling of the polymer. Thisnecessitates prolonged extraction periods and adds considerably to theproduction cost of forming microporous polymers. In addition, theresulting microporous thermoplastic resin has a low melting or softeningtemperature and low resistance to organic solvents. For example, lowdensity polyethylene softens and loses its shape in boiling water.Furthermore, said polyethylene is soluble in Xylene and similar organicsolvents particularly at elevated temperatures. Such drawbacks greatlydeter forming microporous material from polyethylene since itscommercial usefulness is curtailed by such short comings.

One object of this invention is to provide a process for greatlyincreasing the physical and chemical resistance of microporouspolyolefins to organic solvents.

An additional object of the present invention is to devise a processwhereby extraction of the pore-forming solids can be carried out at anincreased rate at elevated temperatures. Yet another object is toprovide a novel process for decreasing the solubility of microporouspolyolefins in organic solvents. Still another object is to provide abiaxially oriented, crosslinked, polyolefin film having micropores. Afurther object is to provide a crosslinked, heat-shrinkable, microporouspolyolefin film.

Other objects and advantages will become apparent from a reading of thefollowing description of the invention.

States Patent The aforest-ated objects are accomplished by the presentnvention which provides a composition of matter comprismg a polyolefinselected from the group consisting of polypropylene andethylene-vinylco- The present invention also provides a process forforming microporous crosslinked polyolefin which comprises (1) Forming asubstantially homogeneous mixture of normally solid polyolefins selectedfrom the group consisting of polyethylene, polypropylene andethylene-vinyl copolymers, 20-400% by weight of said group member of apore-forming solid and 0.1-10% by weight of said group member of a freeradical generating crosslinking agent,

(2) Shaping the mixture into a desired shape, e.g. sheet or film, havinga thickness of 0.1 to 40 mils at temperatures sufficient to melt thepolyolefin below the gel point of the mixture,

(3) Curing the shaped mixture to form a thermoset polyolefin by heatingthe mixture to at least the decomposition temperature of thecrosslinking agent and (4) Thereafter extracting the pore-forming solidfrom the polyolefin with a suitable solvent at temperatures up to thedegradation temperature of the crosslinked polymer.

The following steps are considered variations of and a part of theprocess of the instant invention.

After step three in the above process, i.e. curing the polyolefin byheating to a higher temperature, it is sometimes desirable and oftenpreferable to orient, uniaxia-lly or biaxially, the polyolefin attemperatures within about 10 C. below its melting point up to its normalextrusion temperature. The addition of such a step to the process aidsin removing the unetfected pore-forming solid from its expandedsurroundings. After removal of the poreforming solid, the orientedporous film can be reheated to heat shrink the film. Alternatively, itis possible to perform the orientation step after dissolving out theporeforming solid to obtain a heat shrinkable film on reheating. Thismethod has the advantage that the temperature employed during theextraction of the pore-forming solid can be elevated to the normalextrusion temperature of the crosslinked polyolefin without effectingsubsequent orientation and heat shrinkability.

A further innovation of the instant invention is the use of irradiationinstead of chemical means to crosslink the polyolefin.

The invention further provides the crosslinked, microporous,polyolefin-containing product and the crosslinked, heat-shrinkable,microporous, polyolefin-containing product in the form of sheet andfilm.

Although the invention relates to polyolefins selected from the groupconsisting of ethylene, propylene and ethylene-vinyl copolymers, forpurposes of brevity the The invention further provides the crosslinked,microporous polyethylene product in the form of sheet and film.

As used herein the term pore-forming solid means any material 0.1 tomicrons in diameter which can be extracted from the polyethylene byselected solvents or by vacuum at a temperature below the degradationtemperature of the crosslinked polyethylene. Heretofore extraction ofthe pore-forming solid from thermoplastic polyethylene was of necessityperformed with suitable solvents at temperatures below the softeningpoint of the polymer to avoid loss of shape of the polymer and swellingof the polymer which caused blocking of the pores. By the practice ofthis invention, one is able to extract the pore-forming solids attemperatures above the crystalline melting point, is. up to thedegradation temperature of the polymer due to the fact that thepolyethylene is now thermoset. Such high temperature extraction allowsone to perform the extraction step at a greatly increased rate withoutfear of distorting the final shape of the polymer.

Similarly as used herein the term selected solvents means a vacuum orany solvent that will dissolve the pore-forming solid at a temperaturebelow the degradation temperature of the crosslinked polyethylene.Examples of suitable pore-forming solids and selected solvents thereforare almost endless and will be immediately evident to those skilled inthe art. Pore-forming solids and selected solvents therefor include, butare expressly not. limited to, the examples in the following listing:

Pore-forming solids:

Sugar; sodium chloride; N32CO3 Sodi- Selected solventsDimethylterephthalate;

anthracene Vacuum at 20-200 C.

The term gel point" as used herein means the point at which a threedimensional network of molecules is formed in the polymer. Whencrosslinking by chemical means using a free radical generatingcrosslinking agent, the gel point means those temperatures at whichthere is suflicient decomposition of the crosslinking agent to cause gelformation i.e., a three dimensional network of molecules in the polymer.The gel point of any specific mixture depends upon numerous factors,including e.g., the particular polyolefin, the particular crosslinkingagent, the amount of crosslinking agent, and the half-life of thecrosslinking agent at various shaping temperatures. As the gel point ofany specific mixture is dependent on so many factors, it is bestdetermined by empirical methods, e.g., by extruding a small sample andobserving or determining whether any gel formation is present. In acontinuous operation the shaping step in the instant invention iscarried out prior to crosslinking the polymer at temperatures whichpreclude gel formation in the polymer thus insuring against plugging ofthe shaping mechanism such as an extruder. Subsequent to shaping, thetemperature of the shaped mixture is elevated to the gel point. In theexamples herein where individual or batch samples in the form of filmare processed, it is possible, if desired, to substantially shape andchemically crosslink simultaneously.

As used herein the term decomposition temperature of the crosslinkingagent means a temperature at which the crosslinking agent has ahalf-life of less than about 1.0 minute and preferably less than about0.5 minute. This can best be determined by one skilled in the artempirically.

Crosslinking agents do not ordinarily have a sharp decomposition point,except possibly at very high temperatures. In the usual case, the agentrequires several minutes to decompose substantially quantitatively, andthe rate of decomposition at a given instant is generally proportional.

to the amount of material. Consequently, the decomposition rate for agiven material at a given temperature can generally be determined by itshalf-life at that temperature. The half-life of any free-radicalgenerating agent can be readily determined by one skilled in the art. Inthe case of peroxides, for example, the determination in volved isdescribed in Doehnert et al., Evaluation of Organic Peroxides on theBasis of Halt'Life Data, Ann. Tech. Management Conf., ReinforcedPlastics Div., Soc. Plastics Ind., Inc., 13, Sect. 1-3, 1-8 (1958);Chem. Abs, 53, 19534i (1959).

Organic compounds capable of generating free radicals which are operableas crosslinking agents in the instant invention include organicperoxygen compounds and azo compounds. Suitable organic peroxygencompounds include but are expressly not limited to dicumyl peroxide,1,2-bis(a-cumylperoxyisopropyl)benzene, l,3- bis a.cumylperoxyisopropyl) benzene, l,4 bis (zx-cumylperoxyiospropyl)benzene,tert-butyl perbenzoate, l2bis (t-butylperoxyisopropyl benzene, 1,3-bis(t-butylperoxyisopropyl)benzene, 1,4-bis(t-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane and 2,5dimethyl-Z,5-di=(tert-butylperoxy) -3-hexyne. Operable azo compoundsinclude 2-phenylazo-2,4-dimethylvaleronitrile,2-p'henylazoisobutyronitrile,2,4,4-trimethylvaleronitrile,2-phenyl-azo-isobutyramide and the like.

The following is a list of a few of the operable crosslinking agents andtheir half-life.

Half-life 1 minute at 190 C.

Crosslinking agent:

Di(tert)-butyl) peroxide Tert-butyl hydroperoxide Dichlorobenzylperoxide 1 minute at 112 C. Tert-butyl peracetate 0.5 minute at 178 C.

Dicumyl peroxide 06 minute at 182 C. Diethyl peroxide 1 minute at 198 C.Di(tert-amyl) peroxide 1 minute at 182 C. Cyclohexyl peroxide 0.5 minuteat 226 C.

2,5 dimethyl 2,5 di (tert butyl-peroxy) hexane 2,5 dimethyl 2,5di-(tert- 0.6 minute at 185 C.

a,u-Azodiisobutyronitrile Azodicyclohexane c a r h o n itrile 2 minutesat 166- C. p hydroxyethylazo 0:,7 dimethylvalero-nitrile 2 minutes at182 C.

The crosslinking agents can be used singly or in combination. It is onlynecessary that the gel point of the mixture be sufiiciently high toenable shaping of the mixture in an extruder or other shaping mechanismat temperatures above the melting point of the polymer withoutcrosslinking occurrying.

Any of the various well-known types of polyethylene can be used inmaking film by the process of this invention. Such polyethylenes includethe branched low-density (i.e. about .010 to about .925) material havingmelting points in the range -110 C., as well as the medium densitymaterials and the newer linear high density (about .950 to .980)materials made by the Ziegler process (TiC1 -Al alkyl catalyst) and thePhillips process (hexavalent chrornia on silica-alumina support). Thelinear polyethylenes have melting points in the range of 1 minute at 230C.v

2 minutes at 132 C.

137 C. and therefore require peroxides (or other freeradical generatingcrosslinking agents) that provide gel points higher than these meltingpoint temperatures.

The crosslinking step to form thermoset polyethylene in addition tobeing accomplished by chemical means can be performed by ionizingradiation of either the particle or electromagnetic type as will beshown hereinafter. Such radiation has an energy of at least about 0.01mev. While the irradiation step to crosslink the polyethylene togelation is preferably carried out using corpuscular particles it isalso operable with X-rays or gamma rays. Thus positive corpuscularparticles such as protons, alpha particles and denterons in addition toelectrons and neutrons are operable to bombard the polymer.

Although the irradiation step in the examples herein used a Van deGraatf electron accelerator as the irradiation source, it should beunderstood that the present invention is not limited thereto. Theparticles may be accelerated to high speeds by means of various voltagegradient mechanisms such as a cyclotron, a Cockroft Walton accelerator,a resonant cavity accelerator, a betatron, a GE resonant transformer, 21synchroton, and the like. Furthermore, particle irradiation may also besupplied from radioactive isotopes or an atomic pile. It is alsopossible to crosslink the polymers by the use of a sensitizer, e.g.benzophonone and U. V. light.

When irradiation is employed instead of chemical means to crosslink thepolymer, the upper limit of the temperature range of the shaping step toput the polymer containing the pore-forming solid in the form of film isno longer limited to the decomposition temperature of the crosslinkingagent. Thus it is possible to perform the shaping operation attemperatures ranging from the melting point of the polymer up totemperatures 150 C. in excess thereof, preferably between the meltingpoint and 75 C. thereabove.

The amount of irradiation necessary to crosslink the polyethylene togelation by the practice of this invention is a dose in the range of 0.1to 100 megarads. Obviously, the dosage is dependent upon the molecularweight of the polymer prior to irradiation with a lesser dosage requiredfor higher molecular weight polymer and vice versa.

The resultant crosslinked, microporous polyethylene product of thepresent invention has many and varied uses. For example, it can be usedto make battery separators and the like. It is especially useful as amaterial having pores which will permit gases, e.g. oxygen, to passtherethrough but which are of such size as to prevent the passage ofwater. Thus the product would be useful as a raincoat or covering suchas a tent or the like which allowed one to breathe oxygen Withoutpermitting the countercurrent passage of rainwater. The product is alsouseful in situations requiring high resistance to solvents at elevatedtemperatures such as a filter sheet.

The following examples are set down to illustrate the invention and arenot to be deemed limiting in scope.

Throughout the instant invention the melt index (MI) of the polymer wasmeasured under the conditions specified in ASTM D 1238-52T.

The density of the polymer was measured under the conditions specifiedin ASTM D 1505-57T.

The percent gel content of the crosslinked film in the instant inventionwas measured by refluxing a weighed sample (approximately 0.5 g.) offilm in a cellulose Soxhlet thimble in xylene (containing 0.3 weightpercent 2,6-ditertiary-butyl4-methyl-phenol, commercially availableunder the tradename Ionol from Shell Oil Corp.) for 24 hours. Theinsoluble portion of the sample after drying was weighed to calculatepercent gel as follows:

weight insoluble sample t otal weight sample X 100 Percent gel ples wasto compound the polyethylene, pore-forming solid and crosslinking agenton a Brabender Plastograph, Banbury Mixer or two-roll mill at atemperature about 10-30 C. above the melting point of the polymer butbelow the gel point for about 5-20 minutes. It is also possible to admixthe reactants, preferably in particulate form, at temperatures below themelting point of the polymer, e.g. 25 C. and thereafter heat the mixtureabove the melting point of the polyethylene to form a homogeneousmixture in the molten polymer. It is also sometimes preferred,especially where there is concern with premature crosslinking in themixing or compounding step, to admix solely the polyethylene and theporeforming solid at temperatures whereat the polymer is molten until ahomogeneous mixture is realized e.g. about 10 minutes, and thereafteradd the crosslinking agent with continued mixing for an additional 1-5minutes. This decreases the possibility of premature crosslinking in thecompounding step since the crosslinking agent is not exposed to anelevated temperature for so long a period. For shaping, samples of themixture, (approximately 10-40 gms. in weight) were pressed into films ofabout 10-20 ml. thickness using a platen press at temperatures whereatthe polymer is molten but below the decomposition temperature of thecrosslinking agent. The film samples were then cured in the press for10-30 minutes at 20,000 psi. at temperatures above the gel point. Aftercuring, the self-supporting film samples were transferred to a tankcontaining a selected solvent for the extraction of the pore-formingsolid. For example, when starch is used as the pore-forming solid, thesolvent in the tank is dilute sulfuric acid maintained at a temperatureof 99-100 C. Due to the ability to employ a high temperature withoutdetriment to the thermoset polymer, the extraction step is shortenedconsiderably. Extraction periods of 10 min. to 3 hrs. are sufiicient toleach the pore-forming material out of the polymer. The polymer is thenwashed to remove the solvent therefrom. Samples of the cured,microporous polyethylene film were taken to calculate the percent gelcontent by the aforestated xylene extraction method. The porosity of thepolymer was characterized by its permeability to oxygen and Water vapor.

In the process where the polymer is crosslinked by irradiation, thecompounding of the polyethylene and pore-forming solid is performed attemperatures whereat the polymer is molten e.g. 10-75 C. above themelting point of the polymer. The homogeneous admixture is shaped e.g.,in the form of sheet or film under pressure on a platen press or in anextruder and cooled. The shaped admixture is irradiated with a dosage of05-100 megarads to crosslink the polymer. The pore-forming solid isextracted with a selected solvent as mentioned supra for the chemicalcrosslinking procedure.

Example 1 100 parts of commercially available polyethylene (density 0.91melt index 2.0) and 30 parts of commerically available powdered canesugar as a pore-forming solid having an average diameter of 2 micronswere compounded on a two roll mill at about C. until a homogeneousmixture was obtained. Films of 1-3 mils thickness were pressed on aplaten press at about 135- C. The films were cooled. Samples of thepressed films were crosslinked by irradiation at dosages of 8 and 12megarads with a Van de Graafi electron generator. The crosslinked filmswere immersed in a 3 /2% solution of sulfuric acid at a temperature of99 C. for 60 minutes to extract the sugar from the polymer. This bathwas followed by a bath of distilled water to remove the acid withsubsequent drying of the film samples. On charactenzation the filmsamples irradiated at a dosage of 8 megarads had a gel content of 59%and those subjected to a dosage of 12 megarads had a gel content of 68%.The crosslinked microporous polyethyiene produced by the aforementionedtreatment contained pores ranging in size from 1 to 4 microns indiameter with an average pore size of about 2 microns. To show thedecreased solubility of the crosslinked microporous product, samplesthereof were immersed in a bath of a mixture of 50% xylene and 50%benzene at 80 C. for 3 hours. Control samples of the same polyethylenewhich had not been subjected to crosslinking were also placed in thesolvent bath. At the end of 3 hours the uncrosslinked polyethylene hadcompletely dissolved in the solvent while the crosslinked microporouspolyethylene showed only swelling and on drying recovered its shape.

Example 2 Example 1 was repeated except that the commercially availablepolyethylene used had a density of 0.96 and a melt index of 0.7. Theextraction of the sugar was performed by immersing the crosslinked filmin a 3 /2% solution of sulfuric acid at a temperature of 99 C. for 90minutes. The resultant microporous product irradiated with a dosage of 8megarads had a gel content of 22% and the sample subjected to a 12megarad dosage had a gel content of 35%. The crosslinked, microporous,high density polyethylene recovered its shape on drying after hours at100 C. in a bath of 100% xylene. The control sample of the polyethylenewhich had not been crosslinked was completely dissolved.

Example 3 Example 1 was repeated except that 100 parts of starch 3-7microns average diameter, was used as the poreforming solid with the 100parts of polyethylene. The resultant crosslinked microporous productrecovered its shape on drying after 5 hours in a bath of 50% xylene and50% benzene at 80 C. A control sample which was not subjected tocrosslinking by irradiation was dissolved in the xylene-benzene bathwithin 3 hours.

Example 4 100 parts of commercially available polyethylene (density0.91; melt index 2.0) and 200 parts of silica gel as a pore-formingsolid having an average diameter of 3-7 microns were compounded on a tworoll mill at about 135 C. until a homogeneous mixture was obtained. 1.0part of 2,5dimethyl-2,5-di(t-butylperoxy)-3- hexyne peroxide in abenzene solution were then added as a crosslinking agent to the mixtureand compounding was continued for 3 minutes. The mixture was removedfrom the two roll mill and samples thereof were pressed on a platenpress into mil thick film at 130 C., followed by a twenty minute cure inthe press at 175 C. and 20,000 p.s.i. pressure. The cured film sampleswere then immersed for 3 hours in a 50% solution of NaOH at atemperature of 99 C. to leach the silica gel out of the polymer. Afterwashing with distilled water, the samples were dried overnight at 50 C.On characterization the crosslinked microporous polyethylene film had agel content of 60% and contained pores ranging in size from 2 to 7microns in diameter. The crosslinked microporous polyethylene filmrecovered its shape on drying after immersion in a bath of 50% xyleneand 50% benzene at 80 C. for 6 hours.

Example 5 Example 4 was repeated except that the commercially availablepolyethylene had a density of 0.96 and a melt index of 0.7 and thecrosslinking agent consisted of 0.75 part of2,5-dimethyl-2,5-di(tert-butylperoxy) 3 hexyne. Characterization showedthat the crosslinked, microporous polyethylene film had a gel content of40% and contained pores ranging in size from 3 to 6 microns in diameter.The resultant crosslinked microporous product was immersed in a bath of100% xylene at 1000 C. After 5 hours the crosslinked microporouspolyethylene showed swelling and on drying recovered its shape.

The following examples show the inclusion of the biaxial orientationstep in the invention.

Example 6 parts of commercially available polyethylene (deusity 0.91;melt index 2.0) and 15 parts of commercially available powdered canesugar as a pore-forming solid having an average diameter of 2 micronswere compounded on a Brabender Plastograph at a temperature of C. untila homogeneous mixture was obtained. The mixture was removed from theBrabender Plastograph and pressed into films of 20 mil a platen press.The films were cooled. Samples of the pressed film were crosslinked byirradiation at a dosage of 8 megarads with a Van de Graafi electrongenerator. The thus irradiated film samples were then biaxially orientedunder 5 lbs. air pressure at a temperature of 120 C. until it had anelongation ratio of biaxially oriented film was accurately weighed andplaced in a water bath at 40 C. for 1 hour to extract the poreformingsolid. The crosslinked, microporous polyethylene produced by theaforementioned treatment contained pores ranging in size from 6 to 10microns in diameter with an average pore size of about 8 microns. Theheatshrinkable film was shrunk by reheating it to 100 C. to shrink thefilm to substantially its original size. Rerneasurement of the pores ofthe film showed that the pores ranged in size from 1 to 3 microns indiameter with an average pore size of about 2 microns.

Example 7 the Brabender and pressed on a platen press at 400 F.

to form 20 mil thickness film. The thus formed film were crosslinked byirradiation and a dosage of 8 megarads with a Van de Graaff electrongenerator. The thus irradiated film samples were then biaxially orientedunder 5 lbs. air pressure until it had an elongation ratio of 3X. Thethus crosslinked, oriented copolymer film was immersed in a 3 /2%solution of sulfuric acid at a temperature of 99 C. for 60 minutes toextract the sugar from the copolymer. This bath was followed by a bathin distilled water to remove the acid film samples. On characterizationthe film samples had a gel content greater than 15%.

in size from 4 to 8 microns in diameter with an average pore size of 6microns. The film sample was reheated at 165 C. to heat shrink the film.Reexamination of the heat-shrunked film showed that the average poresize was about 2 microns.

Example 8 thus crosslinked film was then biaxially oriented under 5pounds air pressure at C. until it had an elongation ratio of 3X. Thecrosslinked films were immersed in a 3 /2% solution of sulfuric acid ata temperature of 99 C. for 60 minutes to extract the sugar from thepolymer. This bath was followed by a bath in distilled water to removethe acid with subsequent drying of the film samthickness at 350 F. on,

4x. A sample of the with subsequent drying of the The crosslinked,biaxially oriented, microporous copolymer contained pores ranging 9pics. On characterization the film samples contained pores ranging insize from 4 to 8 microns and an average pore size of about 6 microns,The film was reheated to 165 C. thereby yielding microporous film havingan average pore size of 2 microns.

On repeating the example except that the crosslinking by irradiation andheating was omitted, the biaxial orientation resulted in an elongationratio of 4X. After extraction of the sugar, the film contained poresranging in size from 6 to 10 microns with an average pore size of about8 microns. On reheating the film to 160 C. the average pore size was 2microns.

Example 9 100 parts of commercially available polyethylene density of0.91; and a melt index 2.0 and 30 parts of commercially availablepowdered cane sugar as a pore-forming solid having an average diameterof 2 microns were compounded on a two roll mill at about 130 C. until ahomogeneous mixture was obtained. Films of 20 mil thickness were pressedon a platen press at about 135 C. Samples of the pressed films werecrosslinked by irradiation at a dosage of 8 megarads with a Van deGraaif electron generator. The crosslinked films were washed with a 3/z% solution of sulfuric acid at a temperature of 99 C. for 60 minutesto extract the sugar from the polymer. The films contained pores rangingin size from 1 to 3 microns in diameter. The films were then biaxiallyoriented at a temperature of 100 C. and lbs. air pressure until the filmhad an elongation ratio of 4X. On characterization the thus biaxiallyoriented crosslinked films contained pores ranging in size from 6 to 10microns in diameter with an average pore diameter size of 8 microns. Onreheating the film to 95 C. the pores ranged in size from 1 to 3 micronsin diameter with an average pore size of about 2 microns.

Example 10 100 parts of commercially available polyethylene having adensity of 0.96 and a melt index of 5.0 and 30 parts of commerciallyavailable powdered cane sugar as a pore-forming solid having an averagediameter of 2 microns were compounded on a Brabender Plastograph at atemperature of 140 C. until a homogeneous mixture was obtained. 1.0 partof 1,4-bis(t-butylperoxyisopropyl) benzene was then added as acrosslinking agent to the mixture and compounding was continued for 2minutes. The mixture was removed from the Brabender and samples thereofwere pressed on a platen press into mil thick film at 150 C. followed bya 20 minute cure in the press at 175 C. and 20,000 p.s.i. pressure. Thecured film samples were then biaxially oriented under 5 lbs. airpressure until they had an elongation ratio of 4X. The biaxiallyoriented films were then immersed in a 3 /2% solution of sulfuric acidat a temperature of 99 C. for 60 minutes to extract the sugar from thepolymer. This bath was followed by a bath in distilled water to removethe acid with subsequent drying of the film samples. On characterizationthe film samples contained pores ranging in size from 4 to 8 micronswith an average pore size of about 6 microns. The films were reheated to130 C. thereby yielding crosslinked microporous film having an averagepore size of 2 microns.

The following examples shows the operability of ethylene-vinylcopolymers in the instant invention.

Example 11 100 parts of a commercially available copolymer ofethylene-vinylacetate sold under the tradename Elvax containing 33Weight percent of vinylacetate, having a melt index of and a density of0.957 and a softening point of 115 C. in accord with ASTM D-28 and 30parts of a commercially available powdered cane sugar as a pore-formingsolvent having an average diameter of 2 microns were compounded on aBrabender Plastograph at a temperature of 140 C. until a homogeneousmixture was obtained. Films of 20 mils thickness were pressed on aplaten press from the thus formed mixture at about 135 C. Samples of thepressed films were crosslinked by irradiation at a dosage of 8 megaradswith a Van de Graaif electron generator. The thus crosslinked films werethen biaxially oriented under 5 lbs. air pressure at C. until they hadan elongation ratio of 3 X. The crosslinked, biaxially oriented filmswere immersed in a El /2% solution of sulfuric acid at a temperature of99 C. for 60 minutes to extract the sugar from the polymer. This bathwas followed by a bath in distilled water to remove the acid withsubsequent drying of the film samples. On characterization the filmsamples contained pores ranging in size from 4 to 8 microns and anaverage pore size of about 6 microns. The film was reheated to C.thereby yielding microporous film having an average pore size of 2microns.

Example 12 Example 11 was repeated except that 100 parts of acommercially available ethylene-ethyl acrylate copolymer sold under thetradename Zetafin having a density of 0.929 and a melt index of 18.5 wassubstituted for the ethylene-vinyl acetate copolymer. Oncharacterization, the irradiated, biaxially oriented film samplescontained pores ranging in size from 4 to 8 microns and an average poresize of about 6 microns in diameter. The film on reheating to 125 C.yielded a crosslinked, microporous film having an average pore size of 2microns in diameter.

What is claimed is:

1. A process for forming crosslinked oriented microporous, polyolefinfilm containing a plurality of pores having an average diameter of 0.1to 10 microns which comprises forming a mixture consisting essentiallyof a polyolefin member of the group consisting of polyethylene,polypropylene and ethylene-vinyl copolymers, 20-400% by weight of saidpolyolefin group member of a finely divided pore-forming solid having anaverage diameter in the range 0.1 to 10 microns and 0.1 to 10% by weightof said polyolefin group member of an organic compound capable ofgenerating free radicals, heating the mixture to above the melting pointof the polyolefin group member but below the decomposition temperatureof the organic compound, shaping the thus heated mixture in the form offilm, crosslinking the shaped mixture by heating to at least thedecomposition temperature of the organic compound, orienting thecrosslinked film at a temperature within about 10 C. below its meltingpoint up to its normal extrusion temperature and thereafter extractingthe pore-formingsolid at a temperature below the degradation temperatureof the crosslinked polyolefin group member.

2. The process according to claim 1 wherein the polyolefin group memberis oriented after extracting the poreforming solid.

3. A process for forming crosslinked oriented micro porous polyolefinfilm containing a plurality of pores having an average diameter of 0.1to 10 microns which comprises forming a mixture consisting essentiallyof a polyolefin member of the group consisting of polyethylene,polypropylene and ethylene-vinyl copolymers and 20 to 400% by weight ofsaid polyolefin group member of a finely divided pore-forming solidhaving an average diameter of 0.1 to 10 microns, heating the mixture toa temperature ranging from the melting point of the polyolefin groupmember up to C. in excess thereof, shaping the heated mixture in theform of film, crosslinking the shaped mixture by irradiating withionizing irradiation with a dosage of 0.1 to 100 megarads orienting thecrosslinked film at a temperature within about 10 C. below its meltingpoint up to its normal extrusion temperature and thereafter extractingthe poreforming solid at a temperature below the degradation temperatureof the crosslinked polyolefin group member.

11 12 4. The process according to claim 3 wherein the poly- 3,062,76011/1962 Dermody et a1. 260-2.5 olefin group member is oriented followingthe extraction 3,098,832 7/1963 Pooley 204--159.2 of the pore-formingsolid. 3,144,399 8/1964 Rainer et al. 204l59.2

References Cited 5 MURRAY TILLMAN, Primary Examiner. UNITED STATESPATENTS SAMUEL H. BLECH, Examiner. 2,175,798 10/1939 Hanger 260-25 1. c.BLEU'IGE, Assistant Examiner.

3,055,966 9/1962 Sundberg 260-25

